The goal is to elucidate and understand the fundamental mechanisms underlying the long term photic-neuroendocrine control of prolactin (PRL) cell function in the seasonally-breeding male Syrian hamster (Mesocricetus auratus). The research is also aimed at achieving a better understanding of the pineal gland's role in the mediation of the photoperiodic-neuroendocrine control of PRL cell activity. Initial studies will determine the effects of short photoperiod on PRL synthesis, storage, degradation and secretion during the early stages of gonadal regression. Changes in PRL synthesis will be assessed by measuring the incorporation of 3H-leucine into newly synthesized PRL in vitro as well as by the measurement of in vivo levels of PRL messenger RNA. PRL storage will be determined by measuring the total amount of immunoreactive RIA-PRL in the incubation vials containing pituitaries from each treatment group. PRL release in vivo will be assessed by measuring the amount of immunoreactive RIA-PRL in the serum of hamsters from each treatment group. The degradation of PRL will be assessed via pulse-chase labelling techniques. Other studies will determine the effects of short photoperiod on the release of a PRL-regulatory factor(s) from the hamster stalk median eminence (SME) in vitro. These effects will be correlated with the effects of short photoperiod on PRL cell activity, particularly during the early stages of gonadal regression. Experiments will also be devoted to the determination of whether alterations in PRL cell sensitivity to a PRL-regulatory exposure. Parallel studies will be conducted in which changed in PRL cell sensitivity to known hypothalamic PRL-regulatory factors (dopamine, Gamma amino butyric acid, thyrotrophin-releasing hormone) will be measured in response to short photoperiod. In all of the proposed studies the role of the pineal gland in mediating short photoperiod-induced changes in PRL cell function will be determined by including pinealectomized hamsters. An understanding of the long term inhibitory control of PRL in this species could potentially lead to new approaches to the long term suppression of abnormally high PRL levels in patients with hypothalamic-pituitary disorders and altered reproductive function.